Project Summary We propose to examine the molecular pathways that mediate the species specificity and tropism of one particular poxvirus, namely myxoma virus (MV). MV is a rabbit-specific poxvirus that induces distinctly different disease profiles depending on the specific rabbit species but is nonpathogenic for every other host species, including man. Our lab has extensively studied MV and the disease it causes in rabbits, called myxomatosis, as a model system to investigate the fundamental principles of poxvirus pathogenesis, particularly by exploiting our large and growing collection of targeted MV gene knockout constructs. Several years ago, we unexpectedly discovered that primary mouse cells, which are normally nonpermissive for MV infection, could be rendered fully permissive by interrupting the cellular interferon (IFN) responses. This work then led to the discovery that the majority of human cancer cells tested were fully permissive for MV and that MV is remarkably effective as oncolytic therapy for the treatment of human cancer xenografts in a variety of animal models. We also discovered that two viral host range factors (M-T5 and M063) were also critical for permissive MV replication in many human cancer cells and we could manipulate viral permissiveness and oncolysis through these viral proteins and their host cell protein targets. Finally, our most recent observation related to this proposal is that primary human cells are protected from MV infection synergistically by IFN and tumor necrosis factor (TNF). We propose to: 1- Evaluate the roles of viral host range factors in MV tropism for human cancer cells. We propose to analyze the interactions of MV host range proteins with host cell signaling molecules (like Akt) by mutagenesis, siRNA knockdowns, signaling inhibitors and protein microarrays. We will test a number of adjunct strategies, like signaling modifier drugs used for cancer chemotherapy in man, to increase MV oncolytic potential for a wider spectrum of human cancer cells. 2- Investigate the role of host IFN and TNF responses in MV tropism. We will study MV knockout viruses that are deleted in several key viral host range genes to investigate their roles in inhibiting IFN or TNF responses, and modulating MV tropism and oncolysis. This new information will allow for more rational approaches to optimizing MV virotherapy against a wider spectrum of human cancers, and for controlling MV replication in primary noncancerous human cells.